Such a bone plate is known as the “Metafix,” an implant from Merete Medical GmbH, Berlin. It is used for fixing of the so-called hallux-valgus deformity. It is important here that the toe areas to be fixed are held in a directionally stable manner by the plate. To this end the screws comprise not only a bone screw thread for fixing in the bones in their lower region, but also the head area is provided with an external fine thread that engages into a corresponding internal thread of the screw hole of the plate when the screw has reached the corresponding position when being screwed in. This then aligns it in a directionally stable manner. In order to also fix it in the direction of screwing it is subsequently tightened, as is customary for a screw with a head, during which the conical external thread of the head is fixed in the cylindrical internal thread of the plate.
This has the disadvantage that the region of the fixing of the thread, and therewith the end position of the screw, is precisely determined, so that the adjustment region of the screw, and also that of the plate, is limited. A corresponding implant is also known from U.S. Pat. No. 3,741,205; however, in it smooth pins are used instead of screws, so that the conditions are simplified as regards the screwing-in procedure.
Furthermore, an orthopedic plate system is known from published US Patent Application No. 2004/0073218A1 in which an orthopedic screw with a head comprising a thread should be able to be screwed in in different directions. However, no corresponding counterthread is present in the plate for the head so that it finds no firm hold in it. Moreover, the thread of the screw head becomes scored in the plate, producing material chips, so that the consequence is an endangering of the patient.
A bone screw known from U.S. Pat. No. 6,306,140B1 also comprises a head provided with a thread. However, this screw is not intended to be used with bone plates.